Such a Transmission Electron Microscope (TEM) is known from “Discrimination of heavy and light components in electron microscopy using single-sideband holographic techniques”, K. Downing et al., Optik 42 (1975), page 155-175, and is known as single-sideband imaging.
In a TEM, a sample is imaged by irradiating the sample with a beam of electrons. Often this beam of electrons is a parallel beam. Typically the sample is sufficiently thin for most of the electrons to pass through the sample. Some electrons are elastically scattered by the sample, and leave the sample under another direction than that they entered the sample. These scattered electrons are focused by the objective lens and form in the back-focal plane, of said objective lens, also known as the diffraction plane, a diffraction pattern.
It is noted that each position in the diffraction plane corresponds with a particular angle under which the electrons leave the sample. Therefore the pattern formed in the diffraction plane represents the (Fourier) transform of the image plane to the Fourier plane, For imaging a sample two contrast mechanisms exist: phase contrast and absorption contrast. Phase contrast occurs as a result of the interference of electrons that pass through the sample unhindered with elastically scattered electrons. Phase contrast typically occurs when the sample comprises little heavy atoms and many light atoms, such as carbon, hydrogen, etc. It is noted that in phase contrast the energy of the electrons is hardly changed and it is therefore also referred to as elastic deflection.
In the other contrast mechanism, absorption, electrons are scattered over a much larger angle, as a result of which they are intercepted by, for example, an aperture in the diffraction plane, Some of the electrons are even reflected, resulting in back-scattered electrons. Further some electrons loose energy by e.g. ionizing events, and or not focused in the diffraction plane anymore. All this results in that these electrons do not contribute to the imaging and are commonly referred to as non-elastic deflection.
It is noted that biological samples, polymers, etc., often show phase contrast and little absorption contrast.
The phase contrast of a TEM is often expressed by the Contrast Transfer Function (CTF), which expresses the contrast as a function of the spatial frequency. The CTF in turn can be described as the product of two sub-functions: the envelope function and the Phase Contrast Transfer Function, with both these functions also a function of the spatial frequency. A detailed description of these functions, and their interdependency, is found in “Electron Crystallography of Biological Macromolecules”, R. M. Glaeser et al. (2007), Oxford University Press, ISBN 978-0-19-508871-7, hereby incorporated by reference. More specifically in page 67-72, paragraphs 3.8 and 3.9.
The Phase Contrast Transfer Function is among others a function of the defocus of the lens imaging the sample, and thus of the distance from the sample to the focal plane of the lens. To achieve contrast over a relative wide spatial frequency band users of a TEM often operate at the so-called Scherzer defocus. Glaeser describes the well-known Scherzer defocus (see also FIG. 3.4 of said publication) and the resulting phase contrast transfer function (see e.g. FIG. 3.5 of said publication). Multiplication of the phase contrast transfer function with the envelope function as described in pages 69-72, paragraph 3.9 results in the CTF.
It is noted that for high frequencies the phase contrast transfer function shows oscillations between +1 and −1, and therefore the CTF shows similar oscillations. At which frequency the first zero crossing occurs, depends among others on the distance of the sample to the focal plane of the imaging lens. The Scherzer defocus is often used as it shows a large frequency band where the CTF is continuously positive (above zero). For modern TEM's and biological samples the PCTF, the first zero crossing at Scherzer defocus is typically at a spatial frequency above 3 nm−1, corresponding with a resolution in the image of 0.3 nm (3 Ångstrom). Such a resolution is typically considered sufficient for biological imaging.
As well-known to the person skilled in the art, and as shown in the before mentioned literature, the CTF is low for low spatial frequencies. This implies that in images of a sample showing phase contrast large structures are hard to detect.
In the known method of single-sideband imaging, as described by Downing, half the diffraction plane is blocked (removed) by placing a knife edge in the diffraction plane, covering 50% of the diffraction pattern. As a result of this half of the electrons, the electrons that are scattered such that they are intercepted by the knife edge, cannot interfere with the central beam of undiffracted electrons.
It is noted that single-sideband imaging is also described in “Electron Crystallography of Biological Macromolecules”, R. M. Glaeser et al. (2007), Oxford University Press, ISBN 978-0-19-508871-7, more specifically page 74, paragraph 3.11: ‘Single sideband images: blocking half of the diffraction pattern produces images whose transfer function has unit gain at all spatial frequencies.’
In SSB imaging half of the Fourier space is removed by placing a knife edge in the diffraction plane, covering 50% of the diffraction pattern. By discarding half of the electrons, the contrast is governed by the envelope function only. However, as half of the electrons are discarded, the realized contrast is ‘only’ 50% of the envelope function.
A disadvantage of the single side band method is that the achieved contrast is at best 50% of the envelope.